John King, of the Department of Water Resources, right, checks the snowpack depth during the second manual snow survey of the season at the Phillips station Thursday, Jan. 31, 2019, near Echo Summit, Calif. The survey shows the snow pack at 50 inches deep, with a water content of 18 inches which is 71 percent of average for this location at this time of the year. Statewide the Sierra snowpack is 98 percent of average. (AP Photo/Rich Pedroncelli)

California’s statewide Sierra Nevada snow pack was exactly 100 percent of its historical average on Thursday — precisely normal for this date, with roughly two months left in the winter snow season.

While that might sound mediocre, it’s a big jump from a month ago. On Jan. 1, the snow pack was just 69 percent of its historical average. And a year ago on Jan. 31 it was only 18 percent of normal.

How much snow falls every winter is critical to California’s water picture. The snow, which forms a vast “frozen reservoir” over California’s 400-mile long Sierra mountain range, provides nearly one-third of the state’s water supply for cities and farms as it slowly melts in the spring and summer months, sending billions of gallons of clean, fresh water flowing down dozens of rivers and streams into reservoirs.

It also is key to the state’s ski industry, which suffered significantly during the 2012-16 drought that also caused residential water cutbacks from San Diego to the Bay Area, farm losses as wells and reservoir levels dropped and increased wildfire risk for five years.

“Being at the historical average is good,” said Chris Orrock, a spokesman for the state Department of Water Resources, on Thursday. “We just had a 10-day dry spell, and we’re basically right were we should be, with more snow coming in this weekend.”

Storms forecast for this weekend are expected to bring up to 5 feet of new snow to the Sierra Nevada by Monday.

Widespread rain and mountain snow are expected to begin Friday, with the heaviest precipitation Friday night and Saturday, according to forecasts.

The National Weather Service on Thursday issued a winter storm watch for the Sierra Nevada this weekend. Forecasters are projecting that 1 to 3 feet of new snow will fall across the Sierra. Some areas — like Carson Pass, Ebbetts Pass and Sonora Pass — may get as much as 5 feet by Monday, the weather service said, with poor visibility expected on I-80 and Highway 50 and chain controls likely.

Up to 4 feet of new snow is expected at Donner Summit near Lake Tahoe and along the Tioga Road in Yosemite National Park by Monday.

Every winter, at the beginning of each month, state water officials and other government scientists fan out to take snow measurements at more than 260 sites, with electronic sensors and manual readings. The oldest snow survey location dates back to 1906. One, at Phillips Station, in El Dorado County off Highway 50, south of Lake Tahoe is regularly done with TV cameras and journalists in tow.

On Thursday, the monthly reading at that location, which is near Sierra-at-Tahoe ski resort, showed 98 percent of the historical average. There were 50 inches of snow on the ground — double what there was there a month ago.

“It’s comforting to finally have an average year,” said John Paasch, chief of hydrology and flood control for the Department of Water Resources.

Statewide, on average, the water content of the Sierra Nevada snow was 61 percent Thursday of the April 1 historic average. At the Phillips Station site, it was 71 percent. In other words, California so far this winter has received about two-thirds of a full winter’s Sierra snowfall, with this weekend’s big storms, and all of February and March still to come.

The storms of January also have increased rainfall totals around the state. As of Wednesday evening, Sacramento was at 93 percent of its historical average rainfall for this date, while San Francisco was at 80 percent, Oakland and San Jose were at 76 percent, Los Angeles 139 percent and San Diego 131 percent.

The wet January conditions have caused reservoirs all over the state to rise significantly. Thursday, most of the state’s largest reservoirs were at or above their historical averages for this date. The largest, Shasta, near Redding, was 65 percent full, or 95 percent of normal for this date, while Folsom, near Sacramento was 53 percent full or 102 percent of normal; and San Luis Reservoir, between Gilroy and Los Banos, was 86 percent full or 109 percent of normal.

The weekend storms will bring more than new snow drifts to the Sierra. They’ll also soak the Bay Area with rain.

Meteorologists with the National Weather Service expect the storm to deliver 1 to 2 inches of rain across the Bay Area from Friday night into the weekend. They issued a flash flood watch for Friday night from Sonoma County to Big Sur.

“We expect local flooding along the roads and in small creeks,” said Steve Anderson, a meteorologist with the weather service in Monterey. “Downed trees and power outages. Pretty much a carbon copy of the previous big storm.”